Origin

A crayfish is not a fish but a freshwater crustacean that looks like a small lobster. Its name came into medieval English from Old French crevice, and was probably related to crab (although not to crevice (Middle English), which is from Old French crever, ‘to burst or split’). The spelling was altered in the 16th century simply because people thought that it made more sense: it lives in water, so it must be a fish.